Previously an inactivated RSV vaccine was observed to induce an altered state of reactivity in young vaccinees so that they responded to their first infection with an unusually high frequency. The possibility that live attenuated RSV vaccines might also induce an undesirable state of altered reactivity to disease has been raised on a number of $ occasions. Fortunately, experience during the past few years from a collaborative study with Drs. Parrott and Kim of Children's Hospital of D.C. does not bear out this fear. There was no evidence that 2 live attenuated RSV vaccine strains induced a state of altered reactivity to infection.